Reading your post several times I have to agree with Ted in that the multiport valve seems to have a problem. Doesn't explain the new one doing the same thing though. Ted is also questioning the source of the leak if I read it right. Valve or pump. That should be easy to determine. As for the pump not running up to speed, unlikely. These motors will run at a pretty constant RPM and if you load them to the point of slowing them down then they would overheat quickly. Another point, the pump has nothing to do with a leaking multiport.
Al